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Combinational Logic Circuits – Gates 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 1 Class 4 outline Binary Logic and Gates Basic Gates Implemented Gates Material from section 2-1 of text 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 2 Basic Gates Gates are the base level elements of digital systems. Gates are used to implement Boolean Logic functions. Will start with one level circuits or just a single gate. Then move on to two or more levels of logic circuits. 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 3 Logic gates implementation Logic gates, having two distinct and stable states, high and low, are implemented today using interconnected MOS transistors (most common). Today CMOS – Complementary Metal Oxide Semiconductor – technology is most common. Have had a steady progression of technology used for the implementation of digital circuits. 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 4 Past technologies RTL – Resistor Transistor Logic – implemented with resistors and bipolar transistors TTL – Transistor Transistor Logic – implemented with bipolar transistors NMOS – n-type MOS transistors – implemented with one transistor configured as a MOS resistor – similar in philosophy to RTL 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 5 Basic Functions AND Represented by a dot (·) or the absence of an operator. May be written Z = X AND Y z = x AND y Z=X·Y Z = XY Meaning: Result is TRUE if, and only if, both X and Y are TRUE Verbal– John and Mary went to the dance and to be true, they both went to the dance. 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 6 Basic Functions OR Represented by the + operator symbol Examples Z = X OR Y Z = x OR Y Z=X+Y Meaning: Result is TRUE if at least one of X and Y, or both, are TRUE. Verbal – John or Mary went to the dance. Meaning either John went to the dance, Many went to the dance, or both of them went to it. 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 7 Basic Functions NOT Represented by a bar ¯ over the symbol or the word NOT Examples Z = NOT X Meaning: True if X is False 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU False if X is True 8 Truth table and gate symbols AND function 9/15/09 - L4 Combinational Logic Gates OR function Copyright 2009 - Joanne DeGroat, ECE, OSU 9 Truth table and gate symbols NOT 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 10 But gates are electronic elements And have voltages over time that are interpreted as logic values over time 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 11 Basic gates with more than 2 inputs It is often the case that we have 3 input AND gates or 3 inputs OR gates. Sometimes, even have 4 input gates. Some logic equations have many terms ANDed together. In MOS these many input gates are not readily feasible. Therefore, use multiple levels of logic. 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 12 Class 4 assignment Covered section 2.1 Problems for hand in Problems for practice None today 2-1 do it – This is an important one Section 2.2 9/15/09 - L4 Combinational Logic Gates Copyright 2009 - Joanne DeGroat, ECE, OSU 13